In general, for improving the data rate in communication devices, in particular wireless communication devices, modulation types with non-constant envelopes can be used. However, problems may arise by using high efficient amplifiers for amplifying the signal being transmitted to a suitable transmit power due to the non-constant envelope of the signal.
In FIG. 1, a conventional inphase/quadrature (IQ) transmitter is shown. As can be seen from FIG. 1, one input signal may be the inphase component 2, which can be processed by a digital analogue converter (DAC) 6. In a similar way, as a second input signal, the quadrature component 4 can be processed. Both inphase component 2 and quadrature component 4 can be generated by a suitable signal generator, like a digital signal processor (DSP) or the like (not shown). After passing the low pass filters 8, inphase component 2 can be mixed with a first local oscillator (LO) signal 10 for up converting as well as the quadrature component 4 can be mixed with a second LO signal 12 for up converting. The first LO signal may differ from the second LO signal in a phase shift of ninety degree. Subsequently, the resulting signals can be combined and fed to a programmable gain amplifier (PGA) 14 and to a suitable external power amplifier (PA) 16.
However, such an IQ transmitter has merely a limited signal bandwidth due to the inaccuracy of the provided DACs 6. Another drawback is the large required chip area consumed by the low pass filters 8. Furthermore, complex calibration is needed to set the right corner of the low pass filters 8 in a CMOS process.
A low pass filter can be omitted by implementing a direct digital radio frequency RF modulator instead of the above-proposed IQ transmitter. One possibility is to employ a linear interpolation current-steering DAC to generate base band current signals, which can drive the mixing device directly. For alleviating the still existent problem of limited bandwidth, the DAC and the up conversion mixer can be combined, since the DAC is responsible for the limited bandwidth. Such a modulator device may comprise an array of unit cells driven by a quadrature LO signal and digital IQ data. Another kind of suitable digital RF modulators with a higher power efficiency compared to the previous stated modulators is a digitally envelope-modulated RF modulator. In this modulator, envelope information and phase information can be combined at the RF output.
All the previous stated solutions of prior art comprise the drawback of introduced images or spurs nearby the carrier frequency with offset of the sampling clock frequency and higher order harmonics due to discrete-time to continuous-time conversion. These may violate the spectral mask and the constraints on out-of-band emissions. According to prior art, these undesired spurs can be prevented by filtering the output of the PA. However, extra LC tank and LC resonating circuit respectively as well as frequency tuning and a large chip area are required. Another possibility according to prior art is to increase the sampling frequency and interpolating the input as shown in FIG. 2.
In FIG. 2, a number of unit cell arrays 20 are arranged as an output circuit or output stage each connected to sampling devices 22 via lines 28 and 30. For up converting a data input signal onto a carrier frequency, each unit cell array 20 is supplied with a local oscillator signal or carrier frequency signal via input terminal 18. Oversampling and folding of the data input signal received via terminals 26 can be performed by the unit cell arrays 20 and sampling devices 22. The sampling devices 22 can be supplied with sampling clock signals via terminals 24.1 to 24.4. In the present case, the sampling clock signals applied at the different terminals 24.1 to 24.4 differs in their phase shift. The sampling clock signal is chosen as N times the bandwidth of the base band signal for easy signal processing. Thereby, N is the number of folds used for interpolation or the number of arranged unit cell arrays 20.
However, such a system does not suppress spurs at the offset of ±N, ±2N, ±3N, . . . . While the spurs at the positive offset can be attenuated in a simple manner, in real implementation issues arise due to back folding from the negative complex domain into the real frequency domain. In other words, a spur is caused by folding back the image aliasing from negative frequency offset to a frequency value nearby the carrier frequency. According to prior art, a so called SAW filter can be employed for suppressing these spurs nearby the carrier frequency, which requires a great chip area and its production is connected with high costs.
Additionally, the above stated spurs yield to issues, in case a combination of a transmitter and receiver, which claim different bands, are employed within a communication device. For example, in an application, which uses a Bluetooth transmitter (2.4-2.5 GHz) and a Global System for Mobile Communications (GSM) receiver (1.8-1.9 GHz) simultaneously, the spurs generated by the transmitters may cause undesired peak spurs within the receiver band.
Therefore, it is an object of the present application to provide a digital modulator which prevents undesired spurs nearby the carrier frequency. Another object is to reduce the required chip area of the digital modulator. A further object is to provide a digital modulator which can be produced with reduced costs. Another object is to avoid the implementation of a SAW filter. A further object is to prevent undesired spurs for simultaneously operated different bands.